1. Field of the Invention
This invention relates to an apparatus and method for rapid cooling of a hot bulky workpiece, such as a hot spent anode butt, using a fluidised bed. More particularly, the invention relates to cooling hot spent anode butts while simultaneously reducing emission of hydrogen fluoride gases from the hot butts.
2. Discussion of Background Art
Aluminum metal is produced by electrolysis of alumina dissolved in molten cryolite in "potlines" consisting of numerous reduction cells containing cathodes and carbon anodes. In the so-called pre-baked anode technology, as electrolysis proceeds, the carbon anodes are gradually consumed leaving a residual butt that has to be removed and replaced.
Aluminum smelters of this kind tend to release polluting hydrogen fluoride (HF) into the atmosphere. There are three main sources of such HF emissions, namely:
1. Hydrogen fluoride released from the electrolytic cells; PA1 2. Hydrogen fluoride released from hot anode butts as they are removed from the cells and left to cool, often in an anode butt storage building; and PA1 3. HF emitted from the electrolysis bath taken out of the cell during the anode cavity cleaning and dropped in a container near the electrolytic cell at anode change.
Hot anode butts release HF because they absorb quantities of aluminum fluoride during the electrolysis procedure and the aluminum fluoride reacts with moisture in the air to produce HF, according to the following reaction: EQU 2AlF.sub.3 +3H.sub.2 O 6HF+Al.sub.2 O.sub.3.
When anode butts are originally removed from the electrolysis cells, they are at high temperature of approximately 700.degree. C. (1292.degree. F.) and so the indicated reaction proceeds rapidly in the presence of moist air. Once the surface of the anode butts have cooled to a temperature below about 300.degree. C. (524.degree. F.), however, they tend not to release further HF into the atmosphere.
These hot anode butts are presently cooled in air in large areas within an anode rodding plant. This requires large spaces and intense ventilation because of the evolution of the fluoride gases during the early stage of the cooling. Anode change and butt cooling account for a large fraction of the fluoride emissions from a modern smelter.
U.S. Provisional Patent Application Ser. No. 60/060,848 describes an enclosure system for reducing the generation of fluoride gases by hot anode butts. This is in the form of a container made of a heat and fire-resistant material having an interior volume large enough to accommodate at least one anode butt. When the anode butt is within the container, access to atmospheric air is limited thereby reducing the fluoride gas emissions. This system is typically in the form of a movable unit which acts as a general transport device for hot anode butts.
It is known to use fluidised beds for cooling metal workpieces and one such system is described in German Patent Application 24 55 280, published May 26, 1976. It shows a system for heat treating metal workpieces such as crank shafts, cam shafts, etc. using as the fluidised component fine-grained copper powder.
In Wellwood et al., WO 93/02772, published Feb. 18, 1993, a system is described for scrubbing gaseous fluorides from process exhausts. For this purpose, a fluidised bed of alumina particles in a dry scrubber was used to remove gaseous fluorides from airborne aluminum smelter emissions.
Collet et al., U.S. Pat. 5,182,869, issued Feb. 2, 1993, describes a system for cooling anode rods consisting of a continuous cooling tunnel. The cooling medium was forced air, which was forced through the tunnel.
Anode butts are bulky objects having a relatively low surface area to volume ratio. Because of the bulkiness of the hot butts, it was generally believed that a fluidised bed would not be a suitable cooling medium. Thus, it was believed that because of the large heat source available from the butt interior and the low surface area available to cool the butt, the air being used to create the fluidised bed would cause combustion when the oxygen came in contact with the hot surface and would not be effective in cooling the butt.
It is an object of the present invention to develop a suitable fluidised bed cooling system for hot anode butts.